Thanks for all your responses.
I Have owned my car since 1998, and Lexus had always insisted that the Dunlop SP Sport was the tyre made specifically for the LS400.
Accordingly, I have always replaced like for like with the SP Sports, until now when they are no longer made.
My car has covered 120,000 miles from new, and since it is now 24 years old (93 reg) it is probably worth five bob.
I don't feel like spending over £500 on new tyres, and was hoping for someone to suggest a worthwhile cheap alternative.
There is a KUMHO Solus tyre for £78, and a Black Circles own model for £60 fitted.
Regards